1. Field of the Invention
The present invention relates to a liquid toner for electrophotography and to a method of preparing the liquid toner. More particularly, the invention relates to a liquid toner for electrophotography having a low solidification rate and good redispersion while maintaining the existing physical properties of a liquid toner, such as image density at certain levels. The invention is also directed to a method of preparing the liquid toner having a low solidification rate that can be readily redispersed after phase separation.
2. Description of the Related Art
Generally, an electrophotographic process includes exposing a charged photoconductor to light by irradiating with an image pattern to form an electrostatic latent image on the charged photoconductor, developing a temporary image by contacting the photoconductor with a liquid developer, and transferring the image and liquid developer to a receiver. The final transferring of the image can be directly from the photoconductor or indirectly via an intermediate transporting member. In general, the developed image remains in a permanently molten state by the application of heat and/or pressure.
The electrophotographic process includes a dry developing method and a wet developing method. In the wet developing method, a toner with a particle size on the sub-micron scale can be used, and a high precision image, which is not obtained in the dry developing method, can be obtained. Also, when using the wet developing method, good gradation is attained and it is easy to fix the toner.
A liquid toner used in the wet developing method is a dispersion composed of charge particles known as toner particles and an insulating liquid acting as a carrier. The liquid toner particles include a polymer binder such as an organosol, a colorant such as a pigment or a dye, and a charge controlling agent. This liquid toner is prepared by adding the organosol, the colorant, and the charge controlling agent to the carrier liquid such as a paraffin oil and milling the mixture in an attrition type milling apparatus.
FIG. 1 schematically illustrates the construction of a conventional liquid toner including the organosol. Referring to FIG. 1, the organosol 10, the colorant 11, and the charge controlling agent 12 are dispersed in the carrier liquid. In this case, the organosol 10 acts as a binder; where the organosol 10 is bonded to the colorant 11 to form toner particles.
The liquid toner prepared as described above easily redisperses to its original state by appropriately stirring at room temperature when left for a short period of time. However, toner particles can settle and aggregate and separated from the carrier liquid to form a kind of layer when left for a long period of time, especially at a high temperature. Once aggregated, ink agglomerates do not easily redisperse to its original state even when a strong shearing force is artificially applied thereto.
In addition, when trying to print using the liquid toner that has separated or has agglomerated particles, which is difficult to redisperse as described above, it is difficult to introduce the liquid toner to a developer, and thus, printing is impossible. Even if printing were possible, an image defect can be caused by the agglomerated liquid toner particles that are not sufficiently redispersed. Also, a serious problem may be caused during the storage and distribution of the liquid toner.
Attempts have been made to resolve the above problems by using various additives. Japanese Patent Laid-Open Publication Nos. hei 8-220812 and hei 8-220813 disclose a liquid developer for electrophotography, having a soluble alkali dispersed resin and/or acidic dispersed resin dispersed in a carrier liquid.
Although stable charge characteristics of toner particles and improved developing speed can be expected by the addition of the soluble alkali dispersed resin and/or acidic dispersed resin, it is difficult to prepare a liquid toner having good stability during a high temperature storage, such as a low solidification rate, and good redispersion.